Field of the Invention
The present invention relates to a sheet processing apparatus equipped with a lateral displacement correction unit for correcting displacement of a sheet in a width direction orthogonal to a conveying direction.
Description of the Related Art
Conventionally, as an apparatus provided in an image forming system including a printer, a copying machine, a facsimile machine, and so forth, there has been known a sheet processing apparatus equipped with a shift function for shifting a position of every set number of sheets in a width direction orthogonal to a conveying direction to thereby discharge and stack each sheet at a corresponding position on a discharge tray. The sheet processing apparatus equipped with the shift function is required to cause bundles of sheets sorted by the shift function to be stacked on the discharge tray such that the sheets of each bundle are accurately aligned.
As such a sheet processing apparatus, there has been known one in which alignment plates are retracted upward from a discharge tray, and the alignment plates are lowered to a position of a sheet bundle according to the timing of the discharge of sheets onto a discharge tray to thereby align the sheets of each bundle (see e.g. Japanese Patent Laid-Open Publication No. 2006-206331).
However, the above-described prior art suffers from a problem that if a lateral displacement (positional shift in a width direction) of a sheet conveyed from an upstream apparatus is large, the sheet is brought into collision with the alignment plates when it is to be stacked on the discharge tray, which causes degradation of sheet alignment and the quality of a sheet bundle.
FIGS. 16A to 16C are views useful in explaining the problem with the prior art. FIGS. 16A and 16C are views of the discharge tray, as viewed from above, and FIG. 16B is a view of the same, as viewed in a sheet discharging direction.
In general, in a sheet processing apparatus, before sheets P having a predetermined sheet width Z are discharged, a distance between alignment plates 202a and 202b is increased, and in this state, the alignment plates 202a and 202b are held on standby in respective positions each spaced from an associated side edge of each sheet P assumed to have been discharged, by a predetermined distance X. Then, the sheet P is discharged onto a discharge tray 201 with the alignment plates 202a and 202b held in a state spaced from the respective side edges of the sheet by the predetermined distance X. Further, in the sheet processing apparatus, in a case where the center of the currently conveyed sheet P in a width direction orthogonal to a conveying direction (see FIG. 16B) suffers from displacement from an assumed center position of conveyance (see FIG. 16A), the displacement is corrected beforehand by a lateral displacement correction unit.
However, a sheet of a type, such as an OHP sheet, a translucent vellum sheet, or a sheet of a size smaller than A5R, which is not subjected to the lateral displacement correction, is discharged onto the discharge tray without correction of the lateral displacement thereof. For this reason, in a case where a sheet, for which lateral displacement correction cannot be performed, is conveyed from an upstream apparatus, with a large lateral displacement, the sheet sometimes collides with the alignment plate 202a or 202b when discharged onto the discharge tray 201, as shown in FIG. 16C. When the sheet is brought into collision with the alignment plate, the orientation of the sheet changes. This degrades sheet alignment and the quality of a sheet bundle, and sometimes causes a jam.